Control system for use with electrical amplifiers



Aug. 13, 1935. O E, KEALL 2,010,998

CONTROL SYSTEM FOR USE WITH ELECTRICAL AMPLIFIERS Filed Feb. 16, 1952 'T' I 1 l L INVENTOR OSWOLD E. KEALL ATTORNEY Patented Aug. 13, 1935 ,CONTROL SYSTEM FOR USE WITH ELECTRICAL AMPLIFIERS Oswold Edward Keall, Chelmsford, England, as-

signor to Radio Corporation of America, a corporation of Delaware Application February 16, 1932, Serial No. 593,400

. 3 In Great Britain February 16, 1931 I 2 Claims.

This invention relates tocontrol systems for usein electrical amplifiers, and more particularly :to a system for varying the output from one stage topanother of aresistance capacity coupled or 2.; similar (e. g. galvanically coupled) cascade con- .nected thermionic amplifier.

According to this inventioncontrol of the outa varying the amount of said resistance.

The invention is illustrated in the accompanying drawing which shows diagrammatically various modifications in accordance therewith. In the accompanying drawing, t Fig. lillustrates diagrammatically a preferred form of carrying out the present invention;

Fig. 2 illustrates another form of the invention providing a permanent control;

I Fig. 3 illustrates the invention as applied to a pair of directly connected tubes; and, Fig. 4 illustrates indiagrammatic form theinvention applied to indirectly heated cathode "tubes. Referring to Figure 1 which shows one way of carrying out the inyention a resistance-capacity coupled amplifier comprises two valves V1 and V2 connectedin cascade. 'Theanode A1 of the first valve-V is connected to the source of anode potential through a resistance R and also through ;a condenser K to the grid G2 ofthe second valve V2 as in the usual way, the grid G2 being connected to the cathode point through the usual grid resistance GR. Betweenthe common cathodc point and a variable tapping point T on the resistance R is connected a condenser C and the output from the first valve to the second is varied by varying the position of the tapping point T. Preferably the resistance R. is provided with a plurality of tappings, and the position of the. tap

ping point varied by. means of a switch. It will be seen that with this arrangement the alternating voltage input to the second valve will vary according to the tapping in use, increasing as the tapping position includes higher values of resistance between the anode of the first valve and the tap. The valve V2 may if desired be similarly coupled to a succeeding valve (not shown) and so on for any desired or suitable number of cascade connected stages.

Preferably an additional resistance or is connected permanently in series with the resistance R as shown in Figure 2 this additional resistance serving to provide decoupling resistance no matter where the tapping point may be.

If desired and as also shown in Figure 2 the junction point of the resistance R and the additional resistance or may be connected to the.common cathode point through a decoupling condenser Cl. though in the majority of cases this is not necessary. It will be seen that where the valueof the resistance included between the anode and the tapping point is at a maximum i. e. is equal to the required value of anode resistance for the valve in use, the condenser between the tapping point and the cathode automatically acts as a decoupling condenser.

In the modification shown in Figure 3 the invention is illustrated as applied to a galvanically or direct coupled type of cascade connected amplifier employing battery energized triodes. 7

As throughout the figures like references indicate like parts, it is thought that the arrangement of Figure 3 will be obvious from the preceding description relating to Figures 1 and 2. In Figure 3 the battery for energizing the triodes is indicated at B various required potentials being brought from suitable points to a multi-stud switch, the arrangement should be such that the switch arm does not break contact with any. particular stud until it has made contact with the next stud so that the circuit is never broken. If desired the said resistance may be constituted by a so-called continuously variable resistance.

The present invention may be employed in connection'with an amplifier working on any frequency band by suitably choosing the value of the condenser connected between the tapping and the common cathode point. This condenser should be large for low frequencies, but can be reduced as the frequency increases. For very high or radio frequencies, it is desirable to provide the additional decoupling condenser above referred to, so as to minimize external capacity effects due to leads, batteries and so forth".

Since when very low frequencies are to be handled by the amplifier the impedance of the conin a grid circuit.

erally employed in the anode circuit of a valve, the cost of the apparatus necessary for carrying denser C becomes increasingly important this impedance of course increasing with decrease in frequency a practical limit is reached beyond which the size of condenser C necessary to enable zero output from the first of two coupled stages to be obtainedbecomes so great that the condenser is unduly expensive. However, this difiiculty does not arise except in cases in which it may be required to obtain zero output from ,a valve handling very low frequencies. This disadvantage, which is theoretical rather than practical, Will tend to give rise to amplitude distortion over the frequency range when the frequencies handled are low. In these circumstances it is advisable to limit the use of the control to cases in which a reduction to about onefifth of the maximum amplification available provides sufiicient variation.

It has been found that a control in accordance with this invention is very smooth and even in use. In one practical case in which the invention has been employed in an amplifier designed for use in television equipment and in which the tapped resistance was constituted by a wirewound anode resistance the variation of the taps being obtained by means of a multi-stud switch having a switch arm or brush contact so arranged that before connection between said arm or contact and any stud is broken connectionis made to a next adjacent stud no trace of any jump was observed in the image as the switch arm'moved from stud to stud. This result is, of course, very satisfactory since in the case in question the eye and not the ear judges the result, the eye being, of course, a much more critical member than the ear.

'- The invention offers inter alia the following practical advantages in the grid circuit is that in practice such resistances have 'a substantial value of inductance and capacity, so that there is a tendency to decrease the input impedance of the grid circuit. The inherent inductance and capacity of apparatus employed in carrying out the present invention is of less effect upon the magnification, since the tapped resistance is in an anode circuit and not 2. Owing to the lower value of resistance genout the present invention will generally be lower than would be the case in a grid control system.

3. The load characteristic of the valve remains substantially constant for all tapping positions.

The invention is also applicable to circuits employing valves other than triodes and to coupled circuits other than those shown; for example, the invention is also applicable to that known variation of the circuit of Figure 3 in which indirectly heated cathode valves are employed, the

cathode heaters being energized in parallel across a transformer secondary whose primary is connected to a source of alternating current which also supplies energy to a rectifier constituting the source of anode potential, the anodes and cathodes proper being connected to appropriate points on a potentiometer associated with said rectifier.

Having nowparticularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-

1. In combination a vacuum tube having output electrodes a path comprising a source of direct current and a resistance connected to said electrodes and a second path comprising a condenser and the input electrodes of a second vacuum tube connected across a portion of said first path containing said resistance, means for applying a steady diiference of potential between said input electrodes, a fixed connection including a condenser between one of the input electrodes of the second named vacuum tube and a point of said resistance and a variable connection between the' same input electrode of said second named vacuum tube and said resistance said latter connection being adjustable so as to'control the output from the first tube to the second tube.

' 2.121 combination a space discharge device having output electrodes, a path comprising a source of direct current and a resistance in series connecting said electrodes, a second path comprising a condenser and the input electrodes of a second space discharge device connected across a portion of said first path containing said resistance, means for applying a steady difference -of potential between the input electrodes of said second space discharge device, a variable connection between one of the input electrodes of the second named space discharge device and said resistance, said connection being adjustable so asto control the output from the first tube to the second tube and a. by-pass circuit between a point of said resistance and said input electrode said by-pass circuit acting in conjunction with a portion of said resistance as a filter circuit for preventing unwanted coupling between said two 'space discharge devices.

QSWOLD EDWARD KEALL. 

